Method for producing coaxial cable



y 25, 1967 MASAMICHI YOSHIMURA ETAL 3,332,814

METHOD FOR PRODUCING COAXIAL CABLE 2 Sheets-Sheet 1 Filed March 1, 1966y 25,1967 MASAMICHI YOSHIMURA ETAL 3,332,814

METHOD FOR PRODUCING COAXIAL CABLE Filed March 1, 1966 2 Sheets-Sheet 2United States Patent This application is a continuation-in-part ofapplication v Ser. No. 208,375, filed July 9, 1962 and now abandoned.

This invention relates to a method for producing electrical conductorsof the type known as coaxial cables and coaxials. More particularly,this invention relates to a method and an apparatus for producingcoaxial cable of unique construction. This coaxial cable is providedwith an insulating separator made of a relatively brittle, and thereforehard also, foamed synthetic resin. Heretofore, it has not been possibleto make satisfactory coaxial cable having an insulating separator madeof a brittle foamed synthetic resin, because such a separator, due toits brittleness, would crumble when the coaxial cable would be flexed orbent.

In almost all coaxial cables produced at present, polyethylene is usedas the insulating separator between the inner conductor and the outerconductor. The polyethylene is either in the form of a foam or in theform of a solid sheath or disc.

Solid sheaths ofpolyethylene are generally less preferred thanpolyethylene foam or discs because they do not provide as low adielectric constant as the foam or discs. Insulating separators of thedisc type can be made to have extremely low effective dielectricconstants in comparison with the solid sheath type. Disc type insulatingseparators can be made to have dielectric constants on the order of 1.1to 1.2. However, as the polyethylene discs are fitted, with spacestherebetween, onto the center or inner conductor, the disc typeinsulating separator thereby constructed lacks mechanical strength,particularly when the coaxial cable is bent, and therefore coaxialcables provided with such an insulating separator must be handled withgreat care -and are limited in their utility.

In the case of the foamed type polyethylene separator, although it ispossible, by regulating the degree of foaming of the polyethylene, tocause the separator to have a dielectric constant on the order of 1.4, afurther increase in the degree of foaming causes an increase in thesoftness of the foamed polyethylene, whereby the foamed polyethylenebecomes inadequate in mechanical strength to be used as an insulatingseparator. Accordingly, it is not feasible to lower the dielectricconstant further. The lack of mechanical strength herein referred to isespecial ly a lack of radial rigidity which causes the radial dimensionof the insulating separator to decrease when the coaxial cable is bent,thus decreasing its effectiveness.

The use of a relatively hard and brittle material such as polystyrene asthe insulating separator material has been proposed in the past but,because of difficulties in fabrication and use of cables employing sucha material, it has not been successfully reduced to practice.

It is an object of the present invention to provide a method and anapparatus for fabricating a coaxial cable having an insulating separatorof low dielectric constant comparable to that of a polyethylene disctype separator yet having substantial strength and flexibilitycomparable 3,332,814 Patented Ju1y 25, 1967 ice to that of a coaxialcable having a polyethylene sheath type insulating separator.

According to one aspect of the invention, there is provided a method ofmanufacturing coaxial cable comprising advancing longitudinally aconductor, an insulating tape, and an electrically conductive tapespaced from each other along superposed substantially correspondingpaths, distributing on the insulating tape during longitudinal travelthereof a foamable material, forming the electrically conductive tapeinto a tubular configuration in which 'the electrically conductive tapedefines an outer tubular conductor coaxial with the first conductor andcircumferentially thereof defining therebetween a space in which thefoamable material is disposed, continuously applying an insulating covercircumferentially of the tubular conductor during longitudinal travelthereof, and treating the foamable material with heat to foam it and tobond said insulating tape to it, thereby forming it into a continuousinsulator between said conductors.

Apparatus according to the invention for carrying out this methodcomprises a plurality of means to advance longitudinally an electricalconductor, an insulating tape, and an electrically conductive tape alongsuperposed substantially corresponding paths, means to deposit afoamable material on the insulating tape during longitudinal travelthereof, forming means disposed to receive the conductor and tapes forlongitudinal travel therethrough and comprising guide means for formingthe electrically conductive tape into a tubular outer conductor disposedcircumferentially of the first conductor coaxial therewith definingbetween the two conductors a tubular space in which the foamablematerial is disposed longitudinally of the space, means for applying aninsulating cover circumferentially of the outer con-ductor duringlongitudinal travel thereof, means for receiving the tubular outerconductor in a covered condition with the conductor and the foamablematerial internally thereof and for applying heating energy thereonduring longitudinal travel thereof sufficient to foam the foamablematerial and to bond the insulating tape to the foamable material,thereby forming the foamable material into a continuous insulatorbetween the conductors.

According to another aspect of the invention, there is provided a methodof manufacturing a coaxial cable which is particularly suitable for themanufacture of coaxial cables of relatively large diameter. In a firststage, in a manner like that described above but without the use of anouter conductor and an insulating cover, there is made a semi-finishedcable comprising the axial conductor, foamed insulator and insulatingtape. In a second stage, disposing the semi-finished cable in the samemanner as the axial or inner conductor in the first stage, thesemifinished cable is provided in radially outward order with a secondfoamed insulator, a second insulating tape, a tubular conductor, and aninsulating cover. The apparatus comprises a plurality of means toadvance longitudinally an electrical conductor and an insulating tapealong superposed substantially corresponding paths, means to deposit afoamable material on the insulating tape during longitudinal travelthereof and forming means disposed to receive the conductor and tape forlongitudinal travel therethrough and comprising die means for formingthe tape into a tubular insulator disposed circumferentially of theconductor coaxial therewith defining therebetween a tubular space inwhich the foamable material is disposed longitudinally of the space andmeans for receiving this construction and for applying heating energythereon during longitudinal travel thereof suflicient to foam thefoamable material, to form the semi-finished cable. Then, there areprovided means to advance longitudinally the semi-finished cable,another insulating tape and an electrically conductive tape alongsuperposed substantially corresponding paths, means to deposit afoamable material on this other insulating tape during longitudinaltravel thereof, forming means disposed to receive the semi-finishedcable with the other insulating tape and the electrically conductivetape for longitudinal travel therethrough and comprising die means forforming the electrically conductive tape and the other insulating tapeinto a tubular outer conductor superposed on a tubular insulatordisposed circumferentially of the semi-finished cable therewith definingbetween the semi-finished cable and the tubular insulator a tubularspace in which the foamable material is disposed longitudinally of thespace, means for applying an insulating cover circumferentially of theouter conductor during the longitudinal travel thereof' and means forreceiving the tubular outer conductor in a covered condition with thefirst mentioned conductor and the foamable material internally thereofand for applying heating energy thereon during longitudinal travelthereof suificient to foam the foamable material and to bind the otherinsulating tape to the foamable material.

According to still another aspect of the invention, a foamed sheet, forexample of polystyrene, rather than foamable chips may be used. Thefoamed or ,partly foamed sheet is heated by heating means immediatelybefore the forming means or the forming means is provided with heatingmeans to heat the foamed sheet. If the sheet is foamed, i.e. completelyfoamed, then the purpose of the heating is to render the sheet moreformable and to cause the insulating tape to cohere thereto. If thesheet is partly foamed, an additional purpose of the heating is tocomplete the foaming. It will be appreciated that in either instance aheating step subsequent to the forming step is not necessary.

According to yet another aspect of the invention, the supple tape andfoamed sheet may be formed about the inner conductor in a first stage toform a partially fabricated cable and the outer conductor may be formedabout the partially fabricated cable in a second stage and then thethusly formed cable construction completed with a wrapping of tape.

The invention will now be described in detail, with reference to theaccompanying drawings in which:

FIGS. 1 and 2 are cross sectional views showing em- 'bodiments of thecoaxial cable produced by using the method and the apparatus of theinvention;

FIGS. 3A, 3B, 3C, and 4 through 7 inclusive, are schematic diagrams ofthe method and apparatus of the invention, which diagrams are to bereferred to in the description hereinafter of the method and apparatusof fabricating coaxial cables according to the invention.

A suitable construction for coaxial cables of relatively small diameterfabricated by the method and apparatus of the invention, comprises acenter or inner conductor 1, an insulating separator 2 made of arelatively hard and brittle foamed synthetic resin, preferably foamedpolystyrene, which is formed concentrically about the inner conductor 1,a flexible or supple synthetic resin film layer 3 having a softeningpoint which is no higher than the softening point of the foamedsynthetic resin, for example polyethylene film, intimately bonded to theouter surface of the insulating separator 2, an outer conductor 4 formedconcentrically about the outer surface of the layer 3, and an'outercover layer including a wrapping of a shielding tape and a wrapping of abinding tape (FIG. 1).

It is a unique and important feature of the coaxial cable made by themethod and with apparatus of the present invention that the insulatingseparator 2, made of relatively l hard, brittle, foamed synthetic resin,is enclosed on its outer surface by an intimately cohering, thin film 3made of a supple or flexible synthetic resin having a softening pointwhich is no higher than the melting point of the foamed synthetic resinof the insulating separator. If only a brittle foamed synthetic resin,such as polystyrene, were used, it would be extremely fragile, and evena slight bending would cause cracks to develop in this material, whichwould then disintegrate. However, by covering the foamed separator withan intimately cohering supple film layer 3, for example, ofpolyethylene, according to this invention, the fragility of the foamedresin itself is compensated for, whereby cracks do not easily developwhen the cable is subjected to the bending expected in normal use.

A suitable construction for coaxial cables of relatively large diameterfabricated by the method and apparatus of the invention comprises, inconcentric disposition and in the sequence named from the centeroutward, a center or inner conductor 1, an insulating separator 2 madeof a relatively hard, brittle foamed synthetic resin, such as foamedpolystyrene, a flexible or supple synthetic resin thin film layer 3having a softening point which is no higher than and preferably lowerthan the softening point of the foamed synthetic resin, for example, apolyethylene film, cohering intimately with the outer surface of theseparator 2, an insulating separator 2a made of a relatively hard,brittle foamed synthetic resin, for example, polystyrene, the separator2a being formed about and cohering intimately with the outer surface ofthe layer 3, a flexible or supple synthetic resin thin film layer 3ahaving a softening point which is no higher than the softening point ofthe foamed synthetic resin, for example, polyethylene film, coheringintimately with the outer surface of the separator 20!, an outerconductor'4 and an outer cover layer 5 including a shielding tapewrapping and a binding tape wrapping (FIG. 2).

Longitudinal seams of the materials may simply be butt joined and sincethere is a wrapping of tape no sealing of the seams is required.

The coaxial cables illustrated in FIGS. 1 and 2 can be fabricated by themethods and apparatus described below with reference to FIGS. 3A, 3B,3C, 4, 5, 6 and 7,

In the apparatus illustrated in FIGS. 3A and 3B, a flexible syntheticresin tape 6 having a relatively low melting point, for example, apolyethylene tape, is supplied continuously from a feeding means and isrun in a substantially straight line along the process line. First acontinuous supply of adhesive or solvent 7 is applied by spraying ontothe upper surface of the polyethylene tape 6 and then, onto this,relatively hard and brittle synthetic resin chips 8 which have beentreated to be foamable, but have not yet been foamed, for example,foamable polystyrene chips, are uniformly distributed. The foamablepolystyrene chips may be obtained, for example, by steeping conventionalunfoamable polystyrene chips of a size on the order of 0.5 to 1.1 mm.for 24 hours at room temperature in petroleum ether having a boilingpoint of 60 to 65 C. and methylene chloride or in hexane and methanol.Methods of making foamable polystyrene chips are well known, do not perse constitute the present invention and essentially any may be used.Above and along the distributed chips, the center conductor 9 issupplied continuously, while material 10, for example, copper tape, forthe outer conductor is continuously supplied below and along the lowersurface of the polyethylene tape 6.

At the guiding part, the outer conductorlt), the polyethylene tape 6,the polystyrene chips '8 spread over the tape 6 and the center conductor9 are so arranged as to be superimposed in sequence and then they arepassed through a path provided within the forming apparatus. The path ofthe forming part has a wall surface whose curvature gradually increasesso as to decrease the cross sections of the materials as they advance;thereby, the curvature of each material is made to increase gradually inthe cross section so as to surround the conductor 9 as the center. I

The curvature of the wall surface of the forming part graduallyincreases until at the terminal portion of the forming apparatus 11, thewall surfaces constitute a circular hole, and this terminal portion isthe die part. When each of the above referred to materials including thecenter conductor 9 passes through the said die part, the respectivematerial is shaped in a concentric form around the center conductor 9,the outer conductor 10 being shaped in a cylindrical form with bothedges of the conductor tape contacting each other.

Next, this product assembly, after leaving the forming apparatus 13, theintermediate product is heated of shielding tape and binding tape 12.The thusly wrapped intermediate product is then passed through a heatingapparatus 13. During its passage through the heating apparatus 12, theintermediate product is heated to a temperature of from 110 to 120 C.,whereby the unfoamed polystyrene chips 8 are caused to foam, andsimultaneously, the individual chips are caused to cohere intimatelytogether into an integral body to form a layer 'of foamed polystyrene.At the'same time, the polyethylene tape is bonded cohesively onto theouter surfaces of the foamed polystyrene layer.

The heating apparatus 13 has, for example, a cylindrical' passagewayinside of which is installed an electric heating coil to evenly radiateheat from the inner surface of the passageway. The heat generated inthis apparatus is first carried to the polyethylene tape 6 throughbinding tape 12 and outer conductor 10; then it is transmitted topolystyrene chips 8. Therefore, in order to heat the polystyrene chips 8to a temperature of from 110 to 120 C. while the intermediate producttravels through the heating apparatus 13, it is necessary to heat tape 6to a much higher temperature because there exists a temperature gradientbetween outer and inner parts of the products; in other words, theatmosphere outside the cable should be maintained at higher temperature.Consequently, the temperature of the atmosphere to be heated by theheating apparatus 13 should be determined by the length of the heatingapparatus 13 as well as the forwarding speed of the intermediateproduct; in other words, it should be determined by the time duringwhich the intermediate product is exposed to the heated atmosphere inthe heating apparatus. In the case of a product of relatively smalldiameter, a suitable temperature of the atmosphere in this particularexample would be approximately 120 C, at the center part of the paththrough the heating apparatus 13, provided that the forwarding speed ofthe cable is about 0.5 m./min. and the length of the path through theheating apparatus 13 is about 1 meter.

When the foamable material is foamable polystyrene it is generallyundesirable to heat the polystyrene to a temperature higher than about120 C. because a higher temperature causes bubbles to be expelled fromthe polystyrene, preventing the formation of foamed (i.e., cellular)structure, destroying the cellular structure. One of the reasons whypolyethylene is one suitable material for tape 6 is that its softeningpoint is about from 110 C. to 120 C., which corresponds to the foamingtemperature of the polystyrene chips 8, and therefore as the polystyrenefoams the polyethylene softens and therefore closely adheres to theouter surface of the foamed polystyrene layer. Another reason thatpolyethylene may constitute a part of the insulating separator for thecoaxial cable and is quite suitable for such insulating separator isthat it has excellent high-frequency electrical characteristics, e.-g.,low dielectric constant at high frequencies. Still another reason isthat, as polyethylene tape is excellent in resiliency or pliability, itis capable of fully compensating for the fragility of foamedpolystyrene, when it is closely adhered to the outer surface of thefoamed polystyrene layer, and of preventing the polystyrene layer fromcracking even if a bending force is applied to the cable. Any othermaterial having a softening point no higher than the softening point ofthe foamed material and the other characteristics herein mentioned mayalternatively be used.

Finally, by applying an outer covering onto the cable the fabrication ofa coaxial cable as shown in FIG. 1 is completed. The above descriptionrelates to the case wherein unfoarned polystyrene chips are used, butthe same result can be obtained even when an incompletely foamedmaterial, e.g., polystyrene, is used as the foamable material.

FIG. 3B is a schematic diagram, in elevation, and FIG. 3C, in plan,indicating the manner in which the flow of materials in the fabricationline is maintained. A take-up roll 17 applies tension on and takes upthe finished cable at the required speed, and a wind-up device 18 windsup the cable leaving the take-up roll 17. These two devices, as well asother devices mentioned below may be driven by a common driving systemof known type as described below.

The power from a motor 61 is suitably changed in rotational speed by aspeed-change mechanism 62 and transmitted through gears 63 to a maindrive shaft 66, to which gears and sprockets as necessary are fixed. Aportion of the power from the main drive shaft 66 is transmitted throughgears 67 and a friction coupling 68 to drive the wind-up device 18.Another portion of the power is transmitted through gears 65 to drivethe takeup roll 17. Still another portion of the power is trans mitted,for example, through a sprocket wheel fixed to the main drive shaft 66and an endless chain 64, to drive the tape wrapper 16. FIG. 3C is aschematic diagram, in plan view, showing the wind-up device 18 and itsdriving mechanism.

Although the above-described take-up and wind-up means and the relateddriving system are omitted in the drawings of other fabricationapparatuses (FIGS. 4-7) and descriptions thereof, it is to be understoodthat similar means and system are used in each case.

Coaxial cables of relatively large diameters, as exemplified by theembodiment shown in FIG. 2, can be fabricated in two process stages asindicated in FIGS. 4 and 5. In the apparatus indicated in FIG. 4,whereby the first stage of fabrication is accomplished, a flexiblesynthetic resin tape 6 having a relatively low melting point, forexample, a polyethylene tape, is first supplied into the process line,and onto the upper surface thereof, an adhesive or solvent 7 is applied.On top of this, relatively hard and brittle synthetic resin chips 8which have been treated to be foamable, but have not yet been foamed,for example, foamable polystyrene chips, are distributed uniformly.Above and along this line of chips, the center conductor 9 is supplied.A forming tape 14 is supplied below the polystyrene tape 6 and caused toaccompany the foregoing materials through a forming apparatus 11, where,about the center conductor 9, the foamable polystyrene chips 8, thepolyethylene tape 6, and the forming tape 14 are successively formed inthe longitudinal direction into a covering of concentric construction.

As should be apparent from the foregoing description, the differencebetween the process stage of FIG. 4 and the process of FIG. 3A is that,in FIG. 3A, the outer conductor 10 is utilized, while, in FIG. 4, theforming tape 14 is used. In the process of FIG. 4, when the forming tape14 passes through the forming apparatus 11, its

bonded.

The foamable polystyrene chips 8 and the polyethylene tape 6 surroundingthe center conductor 9 are supported by the forming tape 14.Accordingly, it is desirable that the section of the cylindricalpassageway of the heating apparatus shown in FIG. 4 be the same oressentially the same as the section of the die part of the formingapparatus 11. In addition to considering the forwarding speed of thepartly fabricated cable, heating time within the heating apparatus 13,or heating temperature of the atmosphere as explained with reference toFIG. 3A, it is necessary, in this case, to take into consideration thefact that the forming tape 14 has a different heat conductivity from anouter conductor. In the case of a coaxial cable of relatively largediameter, an example of a suitable temperature is about 120 C. in thecenter part of the path, provided that the forwarding speed of the cableis about 0.5 m./rnin. and the length of the path of the heatingapparatus 13 is about 2 meters.

The forming tape 14 may be made of polytetrafi-uoroethylene or otherflexible heat resistant material. However, the forming tape 14, unlikethe outer conductor, cannot maintain the partly fabricated cable inuniform shape as the polyethylene chips 8 foam, and, .itself, is notcaused by the heating in the heating apparatus 13 to melt or to bebonded adhesively to the polyethylene tape 6. Moreover, if the selectedmaterial such as polytetrafluoroethylene itself, does not stick butslides well, the process step of peeling off tape 14 prior to passingthe semifinished cable to the second stage process is facilitated.

The forming tape 14 may be of endless form as illustrated in FIG. 4, inwhich case it is supported movably on rollers 74 and driven by a drivingroll 73, which, in turn, is driven by a motor 71 through gears 72.

After the semi-finished cable leaves the heating apparatus 13, theforming tape 14 is peeled off, and the semi-finished cable enters thesecond stage process, which is indicated in FIG. 5. This process is thesame as that indicated in FIG. 3 except that the center conductor 9 isreplaced by the semi-finished cable or line core 15 fabricated in thefirst stage process. That is, polyethylene tape 6a is fed into theprocess line and is coated on its upper surface with an adhesive orsolvent 7a; foamable polystyrene chip's 8a are distributed uniformly onthe adhesive or solvent 7a; above and along this line of chips 8a, thesemi-finished cable 15 fabricated in the first stage process is fed; theouter conductor material 1001 is fed along the lower surface of thepolyethylene tape 6a; the foregoing materials are passed through aforming apparatus 11a; shielding tape and binding tape 12a are wrappedabout the partly fabricated cable, which is then passed through aheating apparatus 13a; and, finally, an outer cover is applied (notshown) to complete the coaxial cable shown in FIG. 2.

With respect to the temperature of the heating atmosphere in the heatingapparatus 13 for enabling the polystyrene chips 8a to foam in the secondstage process, an exemplary suitable temperature in the case of acoaxial cable of relatively large diameter is about 120 C. in the centerpart of the path provided that the forwarding speed of the cable is setabout 0.5 m./ min. and the length of the heating path of the heatingapparatus 13a is about 2 meters.

At the point in the process where the semi-finished cable 15 isintroduced, an adhesive or solvent maybe applied on the outer surface ofthe semi-finished cable 15, but this step has been ornittedin thedrawing.

In the above described processes, the application of an adhesive orsolvent is for the purpose of preventing the foamable chips on the tapefrom falling off at the time of forming and is not necessary providedthat the chips do not fall off at the time of forming. Typically,benzene (solvent) may be used. Typical suitable adhesives are latexadhesives. However, any solvent or adhesive which will at least lightlyadhere the chips to the tape is satisfactory.

Other embodiments of the method of the present invention will now bedescribed with reference to FIGS. 6 and 7. In the embodiment illustratedin FIG. 6, a relatively hard, brittle foamed synthetic resin sheet 21,for example, foamed polystyrene sheet material, is fed from a supplymeans 22. A flexible synthetic resin tape 57 having a relatively lowmelting point, for example, a polyethylene tape, is fed from the feedingdevice 58 along the under surface of the sheet 21. An electricallyconductive tape 23, for example, copper tape, which will be the outerconductor of the coaxial cable, is fed below and along the lower surfaceof the tape 57 from a supply means 24.

A center conductor material 25, for example, copper wire,

is fed above and along the upper surface of the sheet 21 from a supplymeans 26.

The foregoing materials are passed through a forming apparatus 27 wherethe foamed synthetic resin sheet 21, the synthetic resin tape 57, andthe electrically conductive tape 23 are successively formed to cover thecenter con ductor 25. For this process step, it is necessary to heat thefoamed synthetic resin sheet 21 in the forming apparatus 27 or prior toits entering the apparatus 27 so as to facilitate its forming, and tocause the synthetic resin tape 57 to be cohered to the outer surface ofthe sheet 21. For example, in the case of using a foamed polystyrene asthe foamed synthetic resin sheet 21, a suitable temperature is from toC.

The forming apparatus 27 is of almost identical structure to the formingapparatus 11 described above with the exception that the former isfurther provided with a heating means. This heating means comprises aheater, e.g., an electric heater, as in the heating apparatus 13 or 13aand heats the atmosphere within the forming apparatus 27. When thefoamed synthetic resin sheet 21 is to be heated up to 110-120 C., thetemperature of the heated atmosphere should be far higher than thedesired temperature of the resin sheet 21. And, in the'case of a coaxialcable of relatively small diameter, it is desirable to maintain thetemperature of the atmosphere at 270 C., for example, provided that theforwarding speed of the cable is set atabout 5 m./min. and the length ofthe heating path of the forming apparatus 27 is 30 cm.

Subsequent to the above-described forming step, the product is in theform of a coaxial core 28, which is then wrapped with a binding tape 29.

In the above-described process, in the case wherein, for example, foamedpolystyrene is used, such a resin of up to about a 97% degree ofexpansion (i.e., foamed volume 97% greater than unfoamed volume) can beeasily made, and such a resin has sufficient flexibility for winding onthe supply means 22, for example, a bobbin. In its original state,however, this resin cannot be formed into a cover about the centerconductor along the longitudinal direction. This forming step, however,can be easily carried out by heating this resin to a temperature of forexample from 110 to 120 C., as previously mentioned.

Furthermore, since the foamed synthetic resin sheet 21 and the syntheticresin tape 57 travel together with the electrically conductive tape 23,the foamed synthetic resin sheet 21 is not subjected to tension and,therefore, even though it is heated and assumes an easily formedcondition, it is not longitudinally deformed. Accordingly, it ispossible to attain excellent controllable forming.

In the arrangement of apparatus shown in FIG. 7, which illustrates stillanother embodiment of the invention, a relatively hard and brittlefoamed synthetic resin sheet 31, for example, a foamed polystyrenesheet, is fed into the process line from a supply means 32. A flexiblesynthetic resin tape 55 having a relatively low melting temperature, forexample, polyethylene tape, is fed from the feeding device 56 along theunder surface of the sheet 31. An endless forming tape 33 is supportedand circulated by a circulating mechanism 34 in such a manner that, atone portion thereof, the endless forming tape 33 travels together withthe lower surface of the sheet 31 and with the tape 55 through a formingapparatus 37, thereby functioning as a forming tape. This endlessforming tape 33 is made of such a material as, for example,polytetrafluoroethylene of relatively high melting point. A centerconductor material 35 is fed from a supply means 36, along the uppersurface of the foamed synthetic resin sheet 31, through the formingapparatus 37, which successively forms the foamed synthetic resin sheet31, the synthetic resin tape 55 and the forming tape 33 about the centerconductor 35. In this case, also, similarly as in the case illustratedin FIG. 6, it is necessary to heat the foamed synthetic resin 31 at orjust in front of the forming apparatus 37 so as to facilitate itsforming and to make the synthetic resin tape 55 cohere to the outersurface of the layer of sheet 31, whereby a semi-finished cable 38a isfabricated. The above description relates to the use of a completelyfoamed synthetic resin sheet, but the same steps can be followed andresults be obtained even when an incompletely foamed synthetic resinsheet is used as the foamable material.

The forming apparatus 37 is of construction like that of the formingapparatus27 which is explained above with reference to FIG. 6.

The semi-finished cable 38a fabricated in the above manner is covered bythe forming tape 33 While passing through the forming apparatus 37, butafter it passes through the forming apparatus 37, the forming tape 33parts from the semi-finished cable 38a. An outer conductor material 40is fed from a supply means 51 and is led, together with thesemi-finished cable 38a, into a second forming apparatus 52, where theouter conductor material 40 is formed about the semi-finished cable 38ato produce the almost completely fabricated cable 38 which then leavesthe forming apparatus 52 and is wrapped by a binding tape 39 to resultin the final product.

A centering device 53 for the center conductor 35 is provided in theforming apparatus 37 and is controlled through its coupled relation withan electrostatic capacity measuring instrument 54 which is installed onthe process line in the portion where the semi-finished cable 38a isexposed.

An example of this process is that when a coaxial cable of relativelysmall diameter is to be made and the sheet 31 is of polystyrene and thetape 55 is of polyethylene, the temperature of the atmosphere in theforming apparatus 37 may be 200 C. if the forwarding speed of the cableis set 2 m./min. and the length of the path through the formingapparatus 37 is 30 cm.

The speed of the endless forming tape 33 is adjusted to be the same asor somewhat lower than the take-up speed of the cable. The reason forany retardation of this speed is that the foamed synthetic resin sheet31 in some instances is reduced in sectional area by the formingapparatus 37, and its feeding speed in such instances becomes lower thanthe cable take-up speed.

Should it be desired to employ a process of the last described type tofabricate coaxial cable of relatively large diameter, two stages wouldbe used as described above with reference to embodiments employingfoamable chips. Assuming, for example, that the pliable insulating tapesare of polyethylene and the foam sheets are of polystyrene, an exampleof operating conditions would be as follows: in the first stage, aforwarding speed of the cable of 1 m./min., a length of the heating pathof the forming apparatus 37 of 50 cm., and a temperature of theatmosphere in the heating path of 200 C.; in the second stage, aforwarding speed of the cable of 1 rn./min., a length of the heatingpath of the forming apparatus 37 of 50 cm., and a. temperature of theheating path of 200 C.

The invention will now be further described by reference to thefollowing specific examples:

Example I Following the procedure, temperature and operating speeddescribed above relative to making a coaxial cable of relatively smalldiameter with the use of chips of foamable material, the following arefurther details of the materials and conditions used: insulating tape ofpolyethylene, 14.5 mm. wide and 0.30 mm. thick; copper tape for outerconductor, 14.8 mm. wide and 0.18 mm. thick; foamable polystyrene chips,0.8 mm. in diameter; copper wire of 1.2 mm. diameter as the innerconductor; distribution of the chips on the polyethylene tape, 5 chipsper sq. cm.; outside diameter of the formed outer conductor, 4.8 mm.

Example 11 Following the procedure, temperature and operating speeddescribed above for making a coaxial cable of relatively large diameterwith the use of chips of foamable material, the following are thematerials and conditions used for the first stage: insulating tape ofpolyethylene, 52 mm. wide and 0.4 mm. thick; foamable polystyrene chips,1.0 mm. in diameter; copper wire 10 mm. in diameter as the innerconductor; distribution of the chips on the polyethylene tape, 6 chipsper sq. cm. The conditions and materials used for the second stage were:insulating tape of polyethylene, 75 mm. wide and 0.4 mm. thick; coppertape for outer conductor, 76.6 mm. wide and 0.25 mm. thick; foamablepolystyrene chips, 1.0 mm. in diameter; distribution of the chips on thepolyethylene tape, 7 chips per sq. cm., outside diameter of the formedouter conductor, 24.5 mm.

Example 111 A coaxial cable of relatively small diameter is made byusing the procedure, temperature and operating speed described abovewith reference to FIG. 6, and the following are additional details as tomaterials and conditions: as the foamed material, foamed polystyrenesheet having a degree of expansion of (Le, foamed volume is 90% greaterthan unfoamed volume), a width of 10 mm. and a thickness of 1.0 mm.; asthe insulating tape, polyethylene tape having a width of. 14.5 mm. and athickness of 0.3 mm.; copper tape for the outer conductor, having awidth of 14.8 mm. and a thickness of 0.3 mm. and having an outsidediameter when formed into the tubular outer conductor of 4.8 mm.; copperwire 10 mm. in diameter as the inner conductor.

Example IV A coaxial cable of relatively large diameter is made by theprocedure described above with reference to FIG. 7, however feeding intothe process line each from its own feed means a second insulating tapesuperposed above the outer conductor tape and a second foamedsheetsuperposed above the second insulating tape'and heating the secondfoamed sheet for the second forming step in the same manner as the firstfoamed sheet was heated for the first forming step, to make it formableand to cause the second insulating tape to cohere to it in the secondforming step, whereby the second foamed sheet and the second insulatingtape as well as the outer conductor tape are formed as coaxial tubularsheaths about the partly fabricated cable. Both forming stages areoperated at the speed and temperature referred to above with referencesto FIG. 7 and additional details as to the conditions and materials thatare used are: copper wire of 10 mm. diameter as the center conductor;foamed polystyrene sheet having a 90% degree of expansion, a width of 41mm. and a thickness of 2.5 mm. as the first stage foamed sheet;polyethylene tape having a width of 52 mm. and a thickness of 0.4 mm. asthe first stage insulating tape; foamed polystyrene sheet having a 90%degree of foaming, a width of 60 mm. and a thickness of 2.5 mm. as thesecond stage foamed sheet; polyethylene tape having a width of 75 mm.and a thickness of 0.4 mm. as the second stage insulating tape; coppertape having a width of 76.6 mm. and a thickness of 0.25 mm. as the outerconductor, which has a formed outside diameter of 24.5 mm.

By the practice of the present invention as described in connection withFIGS. 1 through 5, since polystyrene, which itself has an extremely lowdielectric constant, is caused to foam on the center conductor to formthe insulating separator layer of the coaxial cable of the invention,the dielectric constant of this insulating separator layer is as low ason the order of 1.1. Furthermore, polystyrene, which heretofore has notbeen practically used because of difiiculty in its fabrication, iseasily formed as an excellent insulating separator by using it in theform of chips and causing these chips to foam and simultaneously becomemutually bonded.

By the practice of the present invention as described in connection withFIGS. 6 and 7, the forming process step in the forming apparatus offorming the various covering materials on the center conductor isfacilitated by heating. Moreover, since a forming tape is caused tosupport and travel together with the foamed material and the syntheticresin tape during this process step, the foamed material and thesynthetic resin tape are not subjected directly to tension, whereby theyare moved without risk of tearing or undesirable uncontrolledlongitudinal deformation. Furthermore, although the foamed syntheticresin sheet is sometimes reduced in sectional area during this formingprocess, it is possible, by controlling such factors as the feedingspeed of the sheet and the heating temperature, to control thedielectric constant of the insulating layer of the final coaxial cable.Moreover, particularly in the case wherein foamed polystyrene of adegree of foaming as high as about 97% is used, it is possible toproduce an insulating layer having a dielectric constant as low as 1.03.

Furthermore, by the practice of the present invention described in theforegoing disclosure, a fragile material such as foamed polystyrene forthe insulating separator layer is enclosed within an intimately coheringlayer of a pliable material such as polyethylene, whereby it isreinforced, and cracks are prevented from developing in the fragilefoamed material. Accordingly, the finished cable is provided withsubstantial flexibility with respect to bending expected in normal use.

Although this invention has been described with respect to a fewembodiments thereof, it is not to be so limited as changes andmodifications may be made therein which are within the full intendedscope of the invention, as defined by the appended claims.

What we claim and desire to secure by Letters Patent is:

1. A method of manufacturing a coaxial cable com prising, advancinglongitudinally a conductor, an insulating tape, and an electricallyconductive tape spaced from each other along superposed substantiallycorresponding paths, distibuting on said insulating tape duringlongitudinal travel thereof a foamable material, forming saidelectrically conductive tape into a tubular configuration in which saidelectrically conductive tape defines an outer tubular conductor coaxialwith said conductor and circumferentially thereof defining therebetweena space in which said foamable material is disposed, continuouslyapplying an insulating cover circumferentially of said tubular conductorduring longitudinal travel thereof, and treating said foamable materialwith heat to foam it and to bond said insulating tape to it, therebyforming it into a continuous insulator between said conductors, saidinsulating tape being comprised of a synthetic resin having a softeningpoint no higher than the softening point of the foamed synthetic resin.

2. A method of manufacturing coaxial cable according to claim 1, inwhich the foamable material is a foamable synthetic resin.

3. A method of manufacturing coaxial cable according to claim 2.,including during the forming of said outer conductor forming said tapeinto a tubular arrangement internally of said outer conductor with saidsynthetic resin internally of said tubular arrangement.

4. A method of manufacturing coaxial cable according to claim 3, inwhich the synthetic resin of the first tape comprises polyethylene.

S. A method of manufacturing coaxial cable according to claim 4, inwhich said insulating cover comprises a shielding tape wrapping and abinding tape wrapping.

6. A method of manufacturing a coaxial cable according to claim 5, inwhich said foamable synthetic resin comprises foamable polystyrenechips.

7. A method of manufacturing a coaxial cable comprising advancinglongitudinally a conductor, and an insulating tape, distributing on saidinsulating tape during longitudinal travel thereof a foamable material,forming said insulator tape into a tubular configuration in which saidinsulator tape defines an outer tubular insulator coaxial with saidconductor and circumferentially thereof defining therebetween a space inwhich said foamable material is disposed, treating said foamablematerial with heat to foam it and to bond said insulating tape to it,thereby forming it into acontinuous insulator, around said conductor, toform a semi-finished cable, advancing longitudinally the semi-finishedcable, another insulating tape, and an electrically conductive tapespaced from each other along superposed substantially correspondingpaths, distributing on said other insulating tape during longitudinaltravel thereof a foamable material, forming said electrically conductivetape and said other insulating tape into a tubular configuration inwhich said electrically conductive tape and said other insulating t-apedefine an outer tubular conductor superposed on a tubular insulatorcoaxial with said semi-finished cable and circumferentially thereofdefining between the semi-finished cable and the tubular insulator aspace in which said foamable material is disposed, continuously applyingan insulating cover circumferentially of said tubular conductor duringlongitudinal travel thereof and treating said foamable material withheat to foam it and to bond said other insulating tape to it, therebyforming it into another continuous insulator between the firstcontinuous insulator and the tubular conductor, said insulating tapebeing comprised of a synthetic resin having a softening point no higherthan the softening point of the foamed synthetic resin.

8. A method of manufacturing a coaxial cable comprising advancinglongitudinally a conductor, a sheet of foamed material, an insulatingtape and an electrically conductive tape, spaced from each other alongsuperposed substantially corresponding paths, heating said sheet,forming said sheet while in a heated state with said insulating tape andsaid electrically conductive tape into respective concentric tubularsheaths about said conductor, with the foamed material being the innersheath and the electrically conductive tape being the outer sheath, theamount of heating being sufficient to make the sheet deformable and tocause the insulating tape to cohere thereto, and continuously applyingan insulating cover circumferentially of the electrically conductiveouter sheath during longitudinal travel thereof.

9. A method of manufacturing a coaxial cable comprising advancinglongitudinally a conductor, a sheet of foamed material and an insulatingtape, spaced from. each other along superposed substantiallycorresponding paths, heating said sheet, forming said sheet while in aheated state and said insulating tape into concentric tubular sheathsabout said conductor with said insulating tape being the outer sheath,the amount of heating being sufficient to make the sheet deformable andto cause the insulating tape to cohere thereto thereby making a partlyfabricated cable, advancing longitudinally the partly fabricated cableand an electrically conductive tape spaced from each other alongsuperposed substantially corresponding paths, forming said electricallyconductive tape into a tubular sheath about said partly fabricated cableand continuously applying an insulating cover circumferentially of theelectrically conductive tubular sheath during longitudinal travelthereof.

1 4 References Cited UNITED STATES PATENTS 2,797,731 7/1957 Carlson161--160 X 2,803,730 8/1957 Kinghorn 156-54 2,917,217 12/1959 Sisson156184 3,011,933 12/1961 Barnes et al. 156201 3,144,369 8/1964 Foord eta1. 15651 EARL M. BERGERT, Primary Examiner.

T. R. SAVOIE, Assistant Examiner.

1. A METHOD OF MANUFACTURING A COAXIAL CABLE COMPRISING, ADVANCINGLONGITUDINALLY A CONDUCTOR, AN INSULATING TAPE, AND AN ELECTRICALLYCONDUCTIVE TAPE SPACED FROM EACH OTHER ALONG SUPEROSED SUBSTANTIALLYCORRESPONDING PATHS, DISTIBUTING ON SAID INNSULATING TAPE DURINGLONGITUDINAL TRAVEL THEREOF A FOAMABLE MATERIAL, FORMING SAIDELECTRICALLY CONDUCTIVE TAPE INTO A TUBULAR CONFIGURATION IN WHICH SAIDELECTRICALLY OCONUCTIVE TAPE DEFINES AN OUTER TUBULAR CONDUCTOR COAXIALWITH SAID CONDUCTOR AND CIRCUMFERENTIALLY THEREOF DEFINING THEREBETWEENA SPACE IN WHICH SAID FOAMABLE MATERIAL IS DISPOSED, CONTINUOUSLYAPPLYING AN INSULATING COVER CIRCUMFERENTIALLY OF SAID TUBULAR CONDUCTORDURING LONGITUDINAL TRAVEL THEREOF, AND TREATING SAID FOAMABLE MATERIALWITH HEAT TO FOAM IT AND TO BOND SAID INSULATING TAPE TO IT, THEREBYFORMING IT INTO A CONTINUOUS INSULATOR BETWEEN SAID CONDUCTORS, ANDINSULATING TAPE BEING COMPRISED OF A SYNTHETIC RESIN HAVING A SOFTENINGPOINT NO HIGHER THAN THE SOFTENING POINT OF THE FOAMED SYNTHETIC RESIN.